Recent studies showed that the inner-core structures of hurricanes, like the thermodynamics of the eye and the eyewall as well as the properties of the spiral rainband, critically affect the maximum potential intensity and the intensification of a hurricane (Barnes and Powell 1995<MWR,2348-2368>; Holland 1997<JAS,2519-2541>; Emanuel 1997<JAS,1014-1026>; Willoughby 1998<MWR,in press> and others). Thus, to improve the prediction of hurricane evolution, it is important to simulate accurately the inner- core features and to study the impact of various physical processes (in particular the surface energy fluxes) on these features.
This paper extends the results of the successful 72-h multiscale simulation of hurricane Andrew (Liu et al. 1997<MWR,3073-3093), using a 54/18/6km nested-grid and the NCAR/ PSU non-hydrostatic MM5 model, to simulate explicitly convection and rainbands in the inner-core region (radius < 200 km) with a cloud-resolving grid size (i.e., 2 km). The initial and boundary conditions of the high-resolution model are constructed from the hourly outputs from the coarser domains (18/6km) in a one-way nested-grid mode. The model is integrated for 12 hours from 00 UTC 24 To 12 UTC 24 August 1992, a period which covers the final fast-deepening stage and the landfall stage over Florida of Hurricane Andrew.
The results show that the thermodynamic and dynamic structures of the simulated storm in the 2 km run are similar to the coarser-mesh runs on the vortex scale. However, when compared with radar and other observations, it was found that there are significant improvements on the simulated structures of the inner-core eyewall, the spiral rainband and the organization of convection. For example, the eyewall becomes much more compact and symmetric. The RMW is reduced by 2.5 km and the width of the eyewall almost decreased by half. The spiral rainbands are much better resolved. The model also captures the complex but realistic interaction between the vortex-scale dynamics and the convective motions, resulting in the generation of many interesting scale- interaction features in the regions of the eye and the eyewall. In particular, the strongest convection with a life cycle of < 30 minutes is located in the eyewall but with an intensity much stronger than the coarser resolution runs. The eyewall convection produces pronounced unbalances on the vortex scale, both in the radial and the vertical directions. A very strong potential vorticity (PV) zone is formed along the eye boundary in the deep troposphere. The ring of maximum PV is located right inside the maximum eyewall updraft and it appears to take an active role in the organization of the eyewall convection. A negative PV zone tends to develop right outside the PV core. The outer spiral rainbands are characterized by negative and positive PV patches (< +/- 5 PVU). The convective core in the eyewall region generates inward spiral gust-streaks of ~10 km long. These streaks disturb significantly the vortex-scale surface maximum wind zone, similar to the observation reported by Wakimoto and Black (1994<BAMS,189-200>).
Sensitivity studies have been carried out to examine the impact of several critical factors in the surface and PBL processes on the inner-core structure and the evolution of the intensity of the hurricane. By artificially changing the fluxes of surface momentum, moisture and sensible heat individually in large steps, the effects of varying the energy source (supply vs. consumption) on a simulated mature hurricane were examined. Some very interesting results will be reported at the conference